The present invention relates to an electrophotographic photosensitive device essentially composed of a conductive layer, a photoconductive layer and a transparent insulating layer for an electropohotographic copying machine, and to a method of manufacture thereof.
For the purpose of obtaining an electrostatic latent image, there is already known and commercially employed an electrophotographic process using a photosensitive device essentially consisting of a conductive layer, a photoconductive layer and a transparent insulating layer and comprising for example a primary charging step for subjecting the surface of the photosensitive device to a uniform charging of a predetermined polarity, an exposure step for exposing the surface to a light image, a secondary charging step for applying, simultaneously with the image exposure, an AC corona discharge or DC corona discharge of opposite polarity, and a whole surface exposure step.
One of such known photosensitive device is formed with an aluminium conductive layer, a selenium photoconductive layer and a polyethylene terephthalate (PET) as the insulating layer. Since the selenium based photoconductive layer is P-type, in the above mentioned primary charging step of the electrostatic latent image forming process, sufficient effect to form electric charge pairs across the insulating layer can not be obtained so that electrostatic latent image of sufficiently high electrostatic contrast can not be obtained.
Particularly, during the primary charging step, the dark resistance of the selenium photoconductive layer is high and the injection of carrier from the conductive layer side is low, with the result that the ideal state of forming electric charge pairs only across the insulating layer is not met. Consequently, the intensive charging step effect of the primary charging is decreased, and the contrast between the bright portion and the dark portion depends only on the simultaneous secondary charging and image exposure steps. When the secondary charging is performed by an AC corona charging element, nearly no contrast can be obtained, and when the secondary charging, is performed by a DC corona charging of opposite polarity, only very low contrast can be obtained.
To improve the contrast, an auxiliary method has been proposed wherein the secondary charging step is performed simultaneously with the image exposure steps after the primary charging step and simultaneous with whole surface exposure step or after the primary charging step and whole surface exposure step. While the contrast obtained by this method is improved, it is necessary to provide a whole surface exposure lamp for use in whole surface exposure step after the primary charging step, thereby complicating the apparatus. Further, it is still not possible as a practical matter to obtain sufficient contrast, and in a high speed process, such auxiliary method can not improve the problem.
To improve contrast, it has been proposed that a photoconductive layer formed by powdered material e.g. ZnO or CdS which is scattered in resin binder should be used in lieu of the selenium photoconductive layer. However such a photosensitive device creates problems of humidity durability.